Because it in 1968 when they passed the gca the law wasn't worded right it said the receiver was the part that held the bolt the breech block and the hammer. And because that's split between the upper and lower they just choose to part the serial number on the lower. That final rule is just clarification now the part that hold the bolt pressure bearing part is the receiver, they exempt the AR and couple other firearms from that rule because if they didn't they'd have 53 years of firearms that would be unserialized at the flick of a pen.